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Electronic spectra of jet-cooled cations of hydrogen-bonded complexes of phenol
SpectrochimicaActa, Vol. 50A, No. 819, pp. 1413-1419, 1994
Pergamon OS&M-853!@4)EOO51-B
Copyright @ 1994ElsevierScienceLtd Printedin GreatBritain.All rightsreserved 0584-8539194$7.00+ 0.00
Electronic spectra of jet-cooled cations of hydrogen-bonded phenol SHIN
SATO,
TAKAYUK~ EBATA
complexes of
and NAOHIKO MIKAMI
Department of Chemistry, Faculty of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980, Japan (Received 29 November
1993; accepted 26 January 1994)
Abstmet-Electronic spectra of hydrogen-bonded complex cations of phenol with N(CH,),, NH, and H,O were investigated by photodissociation spectroscopy combined with a new method for the generation of jetcooled molecular complex ions. The method involves the ionization of phenol followed by a jet expansion for the efficient formation of the complex ions as well as for the cooling. Well-resolved vibronic bands, including the bands due to intermolecular stretching vibrations, were obtained for the complex ions of phenol with N(CH3)3and NH,. Differences between the ionic and neutral forms of these complexes were discussed on the basis of the results. INTRODUCTION THE STRUCTURE
of molecular cluster ions is usually different from that of neutral clusters because the ionization induces a large change in intermolecular binding forces among the constituents. In hydrogen-bonded complexes, such a structural change is expected to be large, since the ionization often promotes proton transfer between the constituents. In such cases, spectroscopic study of the molecular cluster ions is of particular importance for their structural analysis. The recent development of laser spectroscopy combined with mass-selecting techniques has contributed to the studies of structure and dynamics of molecular cluster ions [l-3]. In previous papers [4,5], we reported the electronic spectra of hydrogen-bonded complex cations of phenol observed by photodissociation spectroscopy combined with an ion trapping technique. By using this method we identified the characteristic transitions representing the chromophore of the complex ions. It was concluded that the stable form of the phenol-ammonia complex cation is composed of phenoxy radical (C&O a) and NH:, i.e. [C&&O-NH: ] and a similar structure for the trimethyl amine complex, i.e. [C6H50-HN+(CH3),]. These complex ions exhibit the proton-transferred form, while their neutral complexes are known to be the non-proton-transferred form [6,7]. On the other hand, the complexes of phenol with H20 or p-dioxane exhibit the non-proton-transferred form in both the ionic state and the neutral state. In spite of the applicability of the ion trapping technique, however, the spectra were not good enough for a detailed vibrational analysis because of their spectral congestion owing to thermally hot cluster ions. When a large structural change occurs upon the ionization of a cluster, the Franck-Condon region reached by the resonance-enhanced multiphoton ionization (REMPI) is far from the zero point level of the cluster ion. This is the reason for the generation of hot cluster ions. Because of this, the preparation of cold cluster ions is quite difficult, even by the two-colour REMPI technique. The same difficulty appears in zero kinetic energy (ZEKE) or pulsed field ionization (PFI) photoelectron spectroscopies for cluster ions. In this respect it is particularly important to develop a method to overcome the spectral congestion owing to hot cluster ions. In this paper, we report electronic spectra in the visible region of jet-cooled phenol complex ions generated by a new method involving ionization followed by jet-expansion for the complex formation, as well as for the cooling. The spectra of the complex cations of phenol with N(CH3)3 and NH3 show the vibrational structures characteristic of the phenoxy radical chromophore. The intermolecular stretching vibrations involving the hydrogen bond were clearly identified. On the basis of the results, we will discuss the difference between the ionic and the neutral form of these complexes. In contrast, the jet-cooled phenol-H,0 complex ion shows the broad spectrum which is inherent for the transition of the chromophore of the phenol ion. 1413
1414
S.
SATE et
al.
ionization laser Fig. 1. Nozzle system for the generation
of jet-cooled
hydrogen-bonded
complex cation.
EXPERIMENTAL
A gaseous mixture of helium with vapours of phenol and hydrogen-accepting molecules [NHx, N(CH1)3 or HZ01 was used as a supersonic expansion. The nozzle system used in the present work is illustrated in Fig. 1. In front of an orifice of an ordinary pulsed nozzle, a metal tip was attached as an adapter that has a channel orifice of radius 1 mm and length 15 mm. The adapter also has a side hole with a diameter of 2 mm, which crosses the channel at right angles. The crossing point was at 1 mm downstream from the orifice of the pulsed nozzle, where phenol was photoionized. The generation of the jet-cooled complex cations of phenol involves three processes; first, phenol was ionized by REMPI with the resonance at its O-O transition. In the second stage, the complex ion was mostly formed by ion-molecule reaction of phenol ion with hydrogen-accepting molecule in the channel orifice. In the last stage, the complex ions were mostly cooled down in the expansion. A frequency-doubled output of a dye laser pumped by THG of an Nd : YAG laser (Quantel YG580 + Molectron DL-II with Coumarin 540A) was introduced into the side hole and was used for the ionization of phenol in the gaseous stream in the channel. At 20-30 mm downstream from the exit of the channel orifice, the jet-cooled complex cation was excited to the electronic state by a fundamental output of a dye laser pumped by an XeCl excimer laser (Lambda Physik LPX-110 and FL-2002). Since the electronic excitation of the complex ion results in the photodissociation, its electronic spectrum can be obtained as a photodissociation yield spectrum if the dissociation yield is constant with respect to the excitation energy. Thus, the electronic spectrum of the jet-cooled complex cation was obtained by monitoring a specific fragment ion and by scanning the wavelength of the photodissociation laser. The two laser pulses were synchronized with an appropriate delay time of 20-30 ps for the temporal matching between the ionization and the photodissociation. The fragment ions were repelled to an inlet of a Q-pole mass filter, and were detected by an electron multiplier. The ion current was amplified by a current amplifier and was averaged by a digital boxcar integrator system. The ion current intensity was normalized with respect to the laser power, which was simultaneously monitored by a photomultiplier.
RESULTS AND DISCUSSION
Electronic
spectra of [C,HSOH-N(CH3)3]+
and [C,H,OH-(NH,),]+
Figure 2 shows the electronic spectrum of the jet-cooled hydrogen-bonded complex cation, [C6HSOH-N(CH3)3]+, which was obtained by monitoring the fragment ion HN+(CH,), (60 amu). The intense band at 25,357 cm-’ is assigned to the O-O transition of the complex ion, because no intense band is observed in its lower energy side. As was concluded in a previous paper [6], the observed electronic spectrum of the complex ion represents the chromophore of the phenoxy radical. The absorption spectrum of the phenoxy radical in solution was first reported by TRIPATHI and SCHULER [8]. Several theoretical studies [9, lo] predicted an electronic transition of the phenoxy radical in the energy region. The absorption spectrum of the phenoxy radical has been assigned to the transition from the ground state to the second excited state, 2A2+2B2 [lo], so that the spectrum of the complex ion should be assigned to the transition to the second excited state, i.e. the D2tD0 transition. The intense band at 0+ 1101 cm-’ is assigned to a vibronic transition involving the vibration of mode 7a in terms of Wilson notation, as given by TRIPATHI and SCHULER. The mode 7a is the vibration composed of the CO and CC stretches of the benzene ring [ll]. The vibrational frequencies of the vibronic bands
Electronic
25000 Fig. 2. Electronic
26000 27000 Wavenumber ! cm’
spectrum
1415
spectra of complex cations of phenol
of
the jet-cooled [C,H,OH-N(CH,),]
28000
hydrogen-bonded +.
complex
cation,
and their assignments are listed in Table 1. Vibrational modes of the prominent vibronic bands were tentatively assigned on the basis of the similarity of frequencies between the excited and the ground states; the bands at 0 + 531, 799, 881, 994 and 1446 cm-’ were assigned to vibronic transitions involving vibrations of modes 6a, 12, 18u, 1 and 8a of the benzene ring, respectively. Most of the bands in Table 1 were assigned as the fundamentals mentioned above and their combinations. The prominent progression involving the CO mode of 7a shows that the transition is due to the phenoxy radical chromophore. As will be discussed later, we have assigned the low-frequency bands at 0 + 150 and 295 cm-’ as transitions involving an intermolecular vibration; the former is due to the fundamental of the stretching vibration of the hydrogen bond and the latter is its overtone. Figure 3 shows the electronic spectrum of the jet-cooled complex cations of phenol with NH3. Since the clustering of ammonia is efficient, various sizes of the phenol Table 1. Vibrational
V
(cm-‘) 25357 25507 25652 25817 25888 26004 26156 26238 26351 26458 26610 26682 26803 26952 27106 27241 27348 27439 27507 27558 27684 27897 28217 28621
Av (cm-‘) 0
150 295 460 531 647 799 881 994 1101 1253 1325 1446 1595 1749 1884 1991 2082 2150 2201 2327 2540 2860 3264
frequencies
V
(cm-‘) 25387 25591 25820 25918 26035 26194 26270 26383 26491 26712 26849 27020
and assignments of the phenol complex cations
Av (cm-‘)
Assignment
0
%O
204 433 531 648 807 883 996 1104 1325 1462 1633
intermolecular
stretching
CCC bend + ring distortion CCC bend + ring distortion C-H bend ring breath CO stretch + ring distortion
CC stretch
7ai
S. SATE et al.
1416
Wavenumber I cm-’
Fig. 3. Electronic
spectrum
of
the jet-cooled PGHsOH-(NH,
hydrogen-bonded
complex
cation,
)*I+ .
complex cations with (NH,), clusters (n = l-3) were formed. The mass spectrum showed that the dominant species was [C,H,OH-(NH,),]+ (128 amu) and minor species of Th e intensities of the minor two were about [C6H50H-NH31 + and [C&OH-(NH,),]+. one fifth of the dominant one, so the observed electronic transition is mostly due to the complex cation with (NH,),. The feature of the vibronic bands also indicates that the chromophore of the cation is the phenoxy radical. The same chromophore was predicted for [C6HSO-NH; ] in the previous experiment done by the ion trapping method [5]. The frequencies of the observed bands and their assignments are also given in Table 1. The electronic origin is at 25,387 cm-‘, slightly blue shifted (+30 cm-‘) from that of the phenol-N(CH,)3 complex ion. The vibronic bands due to the vibrations of the benzene ring modes correspond well to that of the amine complex ion. The band at 0 + 204 cm-’ is also assigned to the vibronic transition coupled with the intermolecular stretching vibration of the hydrogen bond. Cooling effect
Here we demonstrate one of the merits of the present method for the formation of jetcooled complex ions. In Fig. 4a, a part of the electronic spectrum of the phenol-N(CH,), complex ion near its origin region is reproduced from Fig. 2. Here, phenol was ionized by REMPI via the electronic origin (36,352 cm-‘) of the S,*& transition. Since the adiabatic ionization energy of phenol is 68,6OOcm-’ [6], the two-photon energy for REMPI is about 4100 cm-’ larger than the ionization threshold. Although the photoelectron carries off some of the excess energy as its kinetic energy, most of the energy results
,
I
I,
I
25000
I
I
I,
I
I1
I,
81
I
I,
I
25500 26000 26500 Wavenumber I cm-’
I
I
I,
I
27000
Fig. 4. Electronic spectrum of [C,HSOH-N(CH,)S]+. (a) Jet-cooled hydrogen-bonded cation. (b) Obtained by the ion trap method.
complex
Electronic spectra of complex cations of phenol
1417
in thermal energy, leading to an extremely hot ion. By the present method, however, the cooling of the parent ion and the formation of the cooled complex ion were performed simultaneously by the collisional process in the channel and by the expansion process. Figure 4b shows the same spectral region observed using the ion trapping method reported in our previous paper [4]. In this case, the ionization threshold of the complex was about 56,4OOcm-‘, and the one-colour/two-photon ionization energy was about 71,064 cm-‘, so that the excess energy of the complex ion was more than 14,664 cm-‘. Owing to the large excess energy, an intense hot band (O-126 cm-‘) is seen at the lower energy side of the O-O band. The hot band is due to the intermolecular stretching mode in the Do state. By comparing both spectra, it is evident that the bandwidths are reduced and the intensity of the hot band is definitely diminished in the jet-cooled spectrum. We can estimate the vibrational temperature of the complex ion by knowing the population of the intermolecular vibrational level in the Do state. The intensity of the hot band is determined by the Boltzmann population of the level and by the Franck-Condon (FC) factor of the transition. The FC factor of the hot band is assumed to be the same as that of the cold band, which was obtained from the relative intensity of the band 0 + 150 cm-’ in Fig. 4a. Thus, the vibrational temperature of the spectrum shown in Fig. 4b was estimated to be 300-500 K, while the temperature of the jet-cooled spectrum in Fig. 4a was found to be 120-130 K. The reduction of the vibrational temperature is not sufficient because the expansion condition may be inefficient in the channel nozzle. We will improve the condition by using a higher stagnation pressure and by applying a Lava1 nozzle for the expansion. Electronic spectrum of [C,H,OH-H,O]+
The complex ion of phenol-H,0 was also prepared by REMPI of phenol. The mass spectrum represents only a fragment of the phenol ion and no other ions such as H30+, as reported in a previous paper [5]. Figure 5 shows the electronic spectrum of [C,H,OH-H,O]+ by monitoring the phenol ion as the dissociation product. The spectrum is entirely broad and no vibrational structure appears, in spite of the low temperature condition of the jet-cooled complex ion. The result indicates that the chromophore of the phenol-H,0 complex ion is totally different from that of the complex ion with N(CH3)3 or NH,. The jet-cooled spectrum is quite similar to the spectrum obtained by the ion trapping method reported in our previous paper [5]. Therefore, it is confirmed that the broadness of the spectrum is not due to insufficient cooling. It is well known that such a broad spectrum is characteristic of the CtX transitions of benzene derivative cations [ 12-141. The structureless spectrum may be due to large coupling of the excited state with dissociation continua of the ion. Thus, the jetcooled spectrum shows that the chromophore of the phenol-H,0 complex ion should be the phenol ion and confirms the previous conclusion that the stable form is the complex of phenol ion with H20, i.e. [C6H50H+-H?O].
Fig. 5. Electronic spectrum of [C6H50H-H,O]+. SAtA) 50:8/9-F
S.
1418 Table 2. Frequencies
S~ro
et al.
and force constants of intermolecular Acceptor
stretching vibrations
molecule
W-4 In
NW,)3
H20
”
(cm-‘) States Neutral complex Complex ion t See $ See 5 See 11See 1 See
Ref. Ref. Ref. Ref. Ref.
S” SI Do D2
Y (cm-‘)
(mdynk A-‘)
132t 148t 126 150
0.37 0.47 0.34 0.48
n=l 164$ 1828 -
(mdynkk’) n=2 139 204
n=l
n=2
0.23 0.28 -
0.23 0.48
(ci-‘) 1516 156811 24On -
(mdynkk’) 0.20 0.22 0.51 -
[6]. [7]. [15]. [17]. [16].
Hydrogen-bond
stretching vibrations and the stable forms of the complex ions
Hydrogen-bond stretching vibrations of the phenol complex cations are summarized in Table 2. The stretching vibrations observed in the corresponding neutral complexes are also given, which are taken from the literature [6, 7, 15-171. In Table 2, the following points should be noted; the vibrational frequency of the phenol-N(CH,), complex ion in Do is 126 cm-‘, while it is 132 cm-’ for the S0 neutral complex. Although the comparison is not straightforward in the case of the phenol-ammonia complex, the frequency change is also small; 139 cm-’ in D,, for [C6H50H-(NH3)2]+ and 164 cm-’ for [C6H50H-NH31 in So. On the other hand, a remarkable frequency increase is found in the case of the phenol-H20 complex; the stretching frequency of the complex ion in Do is 240cm-‘, which was observed with photoelectron spectroscopy done by REISER et al. [16], while the corresponding frequency of the S0 neutral complex is 151 cm-’ [15]. In order to compare the hydrogen bond strengths of these complexes and their ions, the force constants are calculated by a simple model for the hydrogen bond. Since the frequency of the intermolecular stretching vibration is much lower than the intramolecular vibrations of the constituents, the stretching mode is assumed to be separable from other modes. This means that the hydrogen bond stretching mode can be represented by a diatomic model. In the model, two mass points, the mass of which corresponds to the molecular weight of the constituents, are vibrating. The calculated force constants of the intermolecular vibrations are also listed in Table 2. The force constants of the complexes in the So and S, states reflect the order of the hydrogen bond strength; phenol-N(CH,),>-(NH,),>-H,O. This is in accord with the order of their proton affinities [l&19]. In the Do states of the complexes, however, the hydrogen bond strength of the phenol-H,0 ion is much larger than that of the amine complex and the ammonia complex in Do. Furthermore, the bond strength of the phenol-H,0 ion increases in comparison to that of So, while the strength of the amine complex and of the ammonia complex in their Do states are much the same as those in S,. It seems that the force field of the stretching vibration of the complex ions with the phenoxy radical chromophore is weaker than that of the complex ions with the phenol ion chromophore. In the phenol-H20 complex, the remarkable increase of the force constant in the ionic state compared to that in the neutral state corresponds well to an increase in stabilization energy; the dissociation energy of the neutral complex in So is 1560 cm-’ [20] and that of the complex ion is estimated to be 6180cm-’ from the ionization threshold of the complex [2]. This result indicates immediately that the hydrogen bond is much stronger in the ionic state than in the neutral state. In the case of the phenol-N(CH,), complex, the stabilization energy in So is 4500 cm-’ and that of the complex ion is estimated to be
Electronic spectra of complex cations of phenol
1419
more than 16,7OOcm-’ [6]. This means that the ionization of the complex greatly stabilizes the hydrogen bond. In this respect, it is anomalous that the force constant of the complex ion is so small. The anomalous change of the intermolecular stretching vibration in the ionic state of the N(CH3)3 complex may be understood in the following way. In the phenol-N(CH,)3 complex ion, there are two dissociation limits; one provides the products of C6H50. and HN+(CH3)3 and the other leads to C6H50H+ and N(CH3)3. The two limits are associated with the dissociation coordinates; the former correlates with the O---NH coordinate and the latter with the OH---N coordinate. Since the stable form of the phenol-N(CH,), the vibrational complex ion has the proton-transferred form, C6H50-HN+(CH3)3, coordinate of the intermolecular stretching mode should be very close to the 0---HN coordinate. Therefore, the force constant of the complex ion is mostly determined by the intermolecular potential gradient associated with the 0-- -HN direction. As given in our previous work [6], the dissociation limit associated with the 0- --HN direction is much lower (-1 eV) than the limit associated with the OH---N direction, which leads to an apparent stabilization energy up to 16,700 cm-‘. This means that the potential well along the 0- - -HN coordinate is shallower than that along the OH---N direction and that the gradient of the potential associated with the 0---HN coordinate should be smaller than the other. Thus, the large structural change involving proton transfer results in a small change in the intermolecular vibrations.
CONCLUSION
The new method for the efficient generation of jet-cooled molecular complex ions is quite useful for the vibrational analysis of their electronic spectra. The phenoxy radical chromophore of the hydrogen-bonded complex cations of phenol-N(CH,), and -(NH,), has been confirmed and the phenol ion chromophore has also been shown in the phenol-H,0 complex ion. The difference in chromophore of the complexes has been discussed on the basis of the intermolecular stretching vibrations observed in the present work.
REFERENCES [1] J. Steadman and J. A. Syage, /. Am. Chem. Sot. 113,6786 (1991); J. Phys. C/rem. %, 9606 (1992). [2] R. J. Lipert and S. D. Colson, J. Chem. Phys. 89,4579 (1988). [3] K. Fuke, H. Yoshiuchi, K. Kaya, Y. Achiba, K. Sato and K. Kimura, Chem. Phys. Left. 108,179 (1984). [4] N. Mikami, T. Sasaki and S. Sato, Chem. Phys. Left. 180,431 (1991). [S] N. Mikami, S. Sato and T. Sasaki, Chem. Phys. Left. 202, 431 (1993). [6] N. Mikami, I. Suzuki and A. Okabe, J. Phys. Chem. 91, 5242 (1987). [7] N. Mikami, A. Okabe and I. Suzuki, J. Phys. Chem. 92, 1858 (1988). [8] G. N. R. Tripathi and R. H. Schuler, J. Chem. Phys. 81, 113 (1984). [9] H. Yu and J. D. Goddard, J. Molec. Strucf. (Theochem.) 233, 129 (1991). [lo] R. Liu and X. Zhou, J. Phys. Chem. 97, 9613 (1993); Chem. Phys. Lett. 207, 185 (1993). [11] H. D. Bist, J. C. D. Brand and D. R. Williams, J. Molec. Spectrosc. 21, 76 (1966). [12] X. Ripoche, I. Dimicoli, J. Le Calve, F. Piuzzi and R. Botter, Chem. Phys. 124, 305 (1988). [13] K. Walter, U. Boesl and E. Schlag, Chem. Phys. Left. 162, 261 (1989). (141 Y. Tsuchiya, M. Fujii and M. Ito, J. Chem. Phys. 90.6965 (1989). [15] R. J. Stanley and A. W. Castleman Jr., J. Chem. Phys. 94, 7744 (1991). [16] G. Reiser, 0. Dopfer, R. Linder, G. Henri, K. Mtiller-Dethlefs, E. W. Schlag and S. D. Colson, Chem. Phys. Left. 181, 1 (1991). [17] T. Ebata, M. Furukawa, T. Suzuki and M. Ito, J. Opt. Sot. Am. B 7, 1890 (1990). [18] D. H. Aue and M. T. Bowers, Gas Phase Ion Chemistry (Edited by M. T. Bowers), Vol. 2. Academic Press, New York (1979). [19] J. A. Syage and J. Steadman, J. Chem. Phys. 95, 2497 (1991). 120) M. Shiitz, T. Biirgi, S. Leutwyler and T. Fischer, J. Chem. Phys. 98, 3763 (1993).
Pergamon OS&M-853!@4)EOO51-B
Copyright @ 1994ElsevierScienceLtd Printedin GreatBritain.All rightsreserved 0584-8539194$7.00+ 0.00
Electronic spectra of jet-cooled cations of hydrogen-bonded phenol SHIN
SATO,
TAKAYUK~ EBATA
complexes of
and NAOHIKO MIKAMI
Department of Chemistry, Faculty of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980, Japan (Received 29 November
1993; accepted 26 January 1994)
Abstmet-Electronic spectra of hydrogen-bonded complex cations of phenol with N(CH,),, NH, and H,O were investigated by photodissociation spectroscopy combined with a new method for the generation of jetcooled molecular complex ions. The method involves the ionization of phenol followed by a jet expansion for the efficient formation of the complex ions as well as for the cooling. Well-resolved vibronic bands, including the bands due to intermolecular stretching vibrations, were obtained for the complex ions of phenol with N(CH3)3and NH,. Differences between the ionic and neutral forms of these complexes were discussed on the basis of the results. INTRODUCTION THE STRUCTURE
of molecular cluster ions is usually different from that of neutral clusters because the ionization induces a large change in intermolecular binding forces among the constituents. In hydrogen-bonded complexes, such a structural change is expected to be large, since the ionization often promotes proton transfer between the constituents. In such cases, spectroscopic study of the molecular cluster ions is of particular importance for their structural analysis. The recent development of laser spectroscopy combined with mass-selecting techniques has contributed to the studies of structure and dynamics of molecular cluster ions [l-3]. In previous papers [4,5], we reported the electronic spectra of hydrogen-bonded complex cations of phenol observed by photodissociation spectroscopy combined with an ion trapping technique. By using this method we identified the characteristic transitions representing the chromophore of the complex ions. It was concluded that the stable form of the phenol-ammonia complex cation is composed of phenoxy radical (C&O a) and NH:, i.e. [C&&O-NH: ] and a similar structure for the trimethyl amine complex, i.e. [C6H50-HN+(CH3),]. These complex ions exhibit the proton-transferred form, while their neutral complexes are known to be the non-proton-transferred form [6,7]. On the other hand, the complexes of phenol with H20 or p-dioxane exhibit the non-proton-transferred form in both the ionic state and the neutral state. In spite of the applicability of the ion trapping technique, however, the spectra were not good enough for a detailed vibrational analysis because of their spectral congestion owing to thermally hot cluster ions. When a large structural change occurs upon the ionization of a cluster, the Franck-Condon region reached by the resonance-enhanced multiphoton ionization (REMPI) is far from the zero point level of the cluster ion. This is the reason for the generation of hot cluster ions. Because of this, the preparation of cold cluster ions is quite difficult, even by the two-colour REMPI technique. The same difficulty appears in zero kinetic energy (ZEKE) or pulsed field ionization (PFI) photoelectron spectroscopies for cluster ions. In this respect it is particularly important to develop a method to overcome the spectral congestion owing to hot cluster ions. In this paper, we report electronic spectra in the visible region of jet-cooled phenol complex ions generated by a new method involving ionization followed by jet-expansion for the complex formation, as well as for the cooling. The spectra of the complex cations of phenol with N(CH3)3 and NH3 show the vibrational structures characteristic of the phenoxy radical chromophore. The intermolecular stretching vibrations involving the hydrogen bond were clearly identified. On the basis of the results, we will discuss the difference between the ionic and the neutral form of these complexes. In contrast, the jet-cooled phenol-H,0 complex ion shows the broad spectrum which is inherent for the transition of the chromophore of the phenol ion. 1413
1414
S.
SATE et
al.
ionization laser Fig. 1. Nozzle system for the generation
of jet-cooled
hydrogen-bonded
complex cation.
EXPERIMENTAL
A gaseous mixture of helium with vapours of phenol and hydrogen-accepting molecules [NHx, N(CH1)3 or HZ01 was used as a supersonic expansion. The nozzle system used in the present work is illustrated in Fig. 1. In front of an orifice of an ordinary pulsed nozzle, a metal tip was attached as an adapter that has a channel orifice of radius 1 mm and length 15 mm. The adapter also has a side hole with a diameter of 2 mm, which crosses the channel at right angles. The crossing point was at 1 mm downstream from the orifice of the pulsed nozzle, where phenol was photoionized. The generation of the jet-cooled complex cations of phenol involves three processes; first, phenol was ionized by REMPI with the resonance at its O-O transition. In the second stage, the complex ion was mostly formed by ion-molecule reaction of phenol ion with hydrogen-accepting molecule in the channel orifice. In the last stage, the complex ions were mostly cooled down in the expansion. A frequency-doubled output of a dye laser pumped by THG of an Nd : YAG laser (Quantel YG580 + Molectron DL-II with Coumarin 540A) was introduced into the side hole and was used for the ionization of phenol in the gaseous stream in the channel. At 20-30 mm downstream from the exit of the channel orifice, the jet-cooled complex cation was excited to the electronic state by a fundamental output of a dye laser pumped by an XeCl excimer laser (Lambda Physik LPX-110 and FL-2002). Since the electronic excitation of the complex ion results in the photodissociation, its electronic spectrum can be obtained as a photodissociation yield spectrum if the dissociation yield is constant with respect to the excitation energy. Thus, the electronic spectrum of the jet-cooled complex cation was obtained by monitoring a specific fragment ion and by scanning the wavelength of the photodissociation laser. The two laser pulses were synchronized with an appropriate delay time of 20-30 ps for the temporal matching between the ionization and the photodissociation. The fragment ions were repelled to an inlet of a Q-pole mass filter, and were detected by an electron multiplier. The ion current was amplified by a current amplifier and was averaged by a digital boxcar integrator system. The ion current intensity was normalized with respect to the laser power, which was simultaneously monitored by a photomultiplier.
RESULTS AND DISCUSSION
Electronic
spectra of [C,HSOH-N(CH3)3]+
and [C,H,OH-(NH,),]+
Figure 2 shows the electronic spectrum of the jet-cooled hydrogen-bonded complex cation, [C6HSOH-N(CH3)3]+, which was obtained by monitoring the fragment ion HN+(CH,), (60 amu). The intense band at 25,357 cm-’ is assigned to the O-O transition of the complex ion, because no intense band is observed in its lower energy side. As was concluded in a previous paper [6], the observed electronic spectrum of the complex ion represents the chromophore of the phenoxy radical. The absorption spectrum of the phenoxy radical in solution was first reported by TRIPATHI and SCHULER [8]. Several theoretical studies [9, lo] predicted an electronic transition of the phenoxy radical in the energy region. The absorption spectrum of the phenoxy radical has been assigned to the transition from the ground state to the second excited state, 2A2+2B2 [lo], so that the spectrum of the complex ion should be assigned to the transition to the second excited state, i.e. the D2tD0 transition. The intense band at 0+ 1101 cm-’ is assigned to a vibronic transition involving the vibration of mode 7a in terms of Wilson notation, as given by TRIPATHI and SCHULER. The mode 7a is the vibration composed of the CO and CC stretches of the benzene ring [ll]. The vibrational frequencies of the vibronic bands
Electronic
25000 Fig. 2. Electronic
26000 27000 Wavenumber ! cm’
spectrum
1415
spectra of complex cations of phenol
of
the jet-cooled [C,H,OH-N(CH,),]
28000
hydrogen-bonded +.
complex
cation,
and their assignments are listed in Table 1. Vibrational modes of the prominent vibronic bands were tentatively assigned on the basis of the similarity of frequencies between the excited and the ground states; the bands at 0 + 531, 799, 881, 994 and 1446 cm-’ were assigned to vibronic transitions involving vibrations of modes 6a, 12, 18u, 1 and 8a of the benzene ring, respectively. Most of the bands in Table 1 were assigned as the fundamentals mentioned above and their combinations. The prominent progression involving the CO mode of 7a shows that the transition is due to the phenoxy radical chromophore. As will be discussed later, we have assigned the low-frequency bands at 0 + 150 and 295 cm-’ as transitions involving an intermolecular vibration; the former is due to the fundamental of the stretching vibration of the hydrogen bond and the latter is its overtone. Figure 3 shows the electronic spectrum of the jet-cooled complex cations of phenol with NH3. Since the clustering of ammonia is efficient, various sizes of the phenol Table 1. Vibrational
V
(cm-‘) 25357 25507 25652 25817 25888 26004 26156 26238 26351 26458 26610 26682 26803 26952 27106 27241 27348 27439 27507 27558 27684 27897 28217 28621
Av (cm-‘) 0
150 295 460 531 647 799 881 994 1101 1253 1325 1446 1595 1749 1884 1991 2082 2150 2201 2327 2540 2860 3264
frequencies
V
(cm-‘) 25387 25591 25820 25918 26035 26194 26270 26383 26491 26712 26849 27020
and assignments of the phenol complex cations
Av (cm-‘)
Assignment
0
%O
204 433 531 648 807 883 996 1104 1325 1462 1633
intermolecular
stretching
CCC bend + ring distortion CCC bend + ring distortion C-H bend ring breath CO stretch + ring distortion
CC stretch
7ai
S. SATE et al.
1416
Wavenumber I cm-’
Fig. 3. Electronic
spectrum
of
the jet-cooled PGHsOH-(NH,
hydrogen-bonded
complex
cation,
)*I+ .
complex cations with (NH,), clusters (n = l-3) were formed. The mass spectrum showed that the dominant species was [C,H,OH-(NH,),]+ (128 amu) and minor species of Th e intensities of the minor two were about [C6H50H-NH31 + and [C&OH-(NH,),]+. one fifth of the dominant one, so the observed electronic transition is mostly due to the complex cation with (NH,),. The feature of the vibronic bands also indicates that the chromophore of the cation is the phenoxy radical. The same chromophore was predicted for [C6HSO-NH; ] in the previous experiment done by the ion trapping method [5]. The frequencies of the observed bands and their assignments are also given in Table 1. The electronic origin is at 25,387 cm-‘, slightly blue shifted (+30 cm-‘) from that of the phenol-N(CH,)3 complex ion. The vibronic bands due to the vibrations of the benzene ring modes correspond well to that of the amine complex ion. The band at 0 + 204 cm-’ is also assigned to the vibronic transition coupled with the intermolecular stretching vibration of the hydrogen bond. Cooling effect
Here we demonstrate one of the merits of the present method for the formation of jetcooled complex ions. In Fig. 4a, a part of the electronic spectrum of the phenol-N(CH,), complex ion near its origin region is reproduced from Fig. 2. Here, phenol was ionized by REMPI via the electronic origin (36,352 cm-‘) of the S,*& transition. Since the adiabatic ionization energy of phenol is 68,6OOcm-’ [6], the two-photon energy for REMPI is about 4100 cm-’ larger than the ionization threshold. Although the photoelectron carries off some of the excess energy as its kinetic energy, most of the energy results
,
I
I,
I
25000
I
I
I,
I
I1
I,
81
I
I,
I
25500 26000 26500 Wavenumber I cm-’
I
I
I,
I
27000
Fig. 4. Electronic spectrum of [C,HSOH-N(CH,)S]+. (a) Jet-cooled hydrogen-bonded cation. (b) Obtained by the ion trap method.
complex
Electronic spectra of complex cations of phenol
1417
in thermal energy, leading to an extremely hot ion. By the present method, however, the cooling of the parent ion and the formation of the cooled complex ion were performed simultaneously by the collisional process in the channel and by the expansion process. Figure 4b shows the same spectral region observed using the ion trapping method reported in our previous paper [4]. In this case, the ionization threshold of the complex was about 56,4OOcm-‘, and the one-colour/two-photon ionization energy was about 71,064 cm-‘, so that the excess energy of the complex ion was more than 14,664 cm-‘. Owing to the large excess energy, an intense hot band (O-126 cm-‘) is seen at the lower energy side of the O-O band. The hot band is due to the intermolecular stretching mode in the Do state. By comparing both spectra, it is evident that the bandwidths are reduced and the intensity of the hot band is definitely diminished in the jet-cooled spectrum. We can estimate the vibrational temperature of the complex ion by knowing the population of the intermolecular vibrational level in the Do state. The intensity of the hot band is determined by the Boltzmann population of the level and by the Franck-Condon (FC) factor of the transition. The FC factor of the hot band is assumed to be the same as that of the cold band, which was obtained from the relative intensity of the band 0 + 150 cm-’ in Fig. 4a. Thus, the vibrational temperature of the spectrum shown in Fig. 4b was estimated to be 300-500 K, while the temperature of the jet-cooled spectrum in Fig. 4a was found to be 120-130 K. The reduction of the vibrational temperature is not sufficient because the expansion condition may be inefficient in the channel nozzle. We will improve the condition by using a higher stagnation pressure and by applying a Lava1 nozzle for the expansion. Electronic spectrum of [C,H,OH-H,O]+
The complex ion of phenol-H,0 was also prepared by REMPI of phenol. The mass spectrum represents only a fragment of the phenol ion and no other ions such as H30+, as reported in a previous paper [5]. Figure 5 shows the electronic spectrum of [C,H,OH-H,O]+ by monitoring the phenol ion as the dissociation product. The spectrum is entirely broad and no vibrational structure appears, in spite of the low temperature condition of the jet-cooled complex ion. The result indicates that the chromophore of the phenol-H,0 complex ion is totally different from that of the complex ion with N(CH3)3 or NH,. The jet-cooled spectrum is quite similar to the spectrum obtained by the ion trapping method reported in our previous paper [5]. Therefore, it is confirmed that the broadness of the spectrum is not due to insufficient cooling. It is well known that such a broad spectrum is characteristic of the CtX transitions of benzene derivative cations [ 12-141. The structureless spectrum may be due to large coupling of the excited state with dissociation continua of the ion. Thus, the jetcooled spectrum shows that the chromophore of the phenol-H,0 complex ion should be the phenol ion and confirms the previous conclusion that the stable form is the complex of phenol ion with H20, i.e. [C6H50H+-H?O].
Fig. 5. Electronic spectrum of [C6H50H-H,O]+. SAtA) 50:8/9-F
S.
1418 Table 2. Frequencies
S~ro
et al.
and force constants of intermolecular Acceptor
stretching vibrations
molecule
W-4 In
NW,)3
H20
”
(cm-‘) States Neutral complex Complex ion t See $ See 5 See 11See 1 See
Ref. Ref. Ref. Ref. Ref.
S” SI Do D2
Y (cm-‘)
(mdynk A-‘)
132t 148t 126 150
0.37 0.47 0.34 0.48
n=l 164$ 1828 -
(mdynkk’) n=2 139 204
n=l
n=2
0.23 0.28 -
0.23 0.48
(ci-‘) 1516 156811 24On -
(mdynkk’) 0.20 0.22 0.51 -
[6]. [7]. [15]. [17]. [16].
Hydrogen-bond
stretching vibrations and the stable forms of the complex ions
Hydrogen-bond stretching vibrations of the phenol complex cations are summarized in Table 2. The stretching vibrations observed in the corresponding neutral complexes are also given, which are taken from the literature [6, 7, 15-171. In Table 2, the following points should be noted; the vibrational frequency of the phenol-N(CH,), complex ion in Do is 126 cm-‘, while it is 132 cm-’ for the S0 neutral complex. Although the comparison is not straightforward in the case of the phenol-ammonia complex, the frequency change is also small; 139 cm-’ in D,, for [C6H50H-(NH3)2]+ and 164 cm-’ for [C6H50H-NH31 in So. On the other hand, a remarkable frequency increase is found in the case of the phenol-H20 complex; the stretching frequency of the complex ion in Do is 240cm-‘, which was observed with photoelectron spectroscopy done by REISER et al. [16], while the corresponding frequency of the S0 neutral complex is 151 cm-’ [15]. In order to compare the hydrogen bond strengths of these complexes and their ions, the force constants are calculated by a simple model for the hydrogen bond. Since the frequency of the intermolecular stretching vibration is much lower than the intramolecular vibrations of the constituents, the stretching mode is assumed to be separable from other modes. This means that the hydrogen bond stretching mode can be represented by a diatomic model. In the model, two mass points, the mass of which corresponds to the molecular weight of the constituents, are vibrating. The calculated force constants of the intermolecular vibrations are also listed in Table 2. The force constants of the complexes in the So and S, states reflect the order of the hydrogen bond strength; phenol-N(CH,),>-(NH,),>-H,O. This is in accord with the order of their proton affinities [l&19]. In the Do states of the complexes, however, the hydrogen bond strength of the phenol-H,0 ion is much larger than that of the amine complex and the ammonia complex in Do. Furthermore, the bond strength of the phenol-H,0 ion increases in comparison to that of So, while the strength of the amine complex and of the ammonia complex in their Do states are much the same as those in S,. It seems that the force field of the stretching vibration of the complex ions with the phenoxy radical chromophore is weaker than that of the complex ions with the phenol ion chromophore. In the phenol-H20 complex, the remarkable increase of the force constant in the ionic state compared to that in the neutral state corresponds well to an increase in stabilization energy; the dissociation energy of the neutral complex in So is 1560 cm-’ [20] and that of the complex ion is estimated to be 6180cm-’ from the ionization threshold of the complex [2]. This result indicates immediately that the hydrogen bond is much stronger in the ionic state than in the neutral state. In the case of the phenol-N(CH,), complex, the stabilization energy in So is 4500 cm-’ and that of the complex ion is estimated to be
Electronic spectra of complex cations of phenol
1419
more than 16,7OOcm-’ [6]. This means that the ionization of the complex greatly stabilizes the hydrogen bond. In this respect, it is anomalous that the force constant of the complex ion is so small. The anomalous change of the intermolecular stretching vibration in the ionic state of the N(CH3)3 complex may be understood in the following way. In the phenol-N(CH,)3 complex ion, there are two dissociation limits; one provides the products of C6H50. and HN+(CH3)3 and the other leads to C6H50H+ and N(CH3)3. The two limits are associated with the dissociation coordinates; the former correlates with the O---NH coordinate and the latter with the OH---N coordinate. Since the stable form of the phenol-N(CH,), the vibrational complex ion has the proton-transferred form, C6H50-HN+(CH3)3, coordinate of the intermolecular stretching mode should be very close to the 0---HN coordinate. Therefore, the force constant of the complex ion is mostly determined by the intermolecular potential gradient associated with the 0-- -HN direction. As given in our previous work [6], the dissociation limit associated with the 0- --HN direction is much lower (-1 eV) than the limit associated with the OH---N direction, which leads to an apparent stabilization energy up to 16,700 cm-‘. This means that the potential well along the 0- - -HN coordinate is shallower than that along the OH---N direction and that the gradient of the potential associated with the 0---HN coordinate should be smaller than the other. Thus, the large structural change involving proton transfer results in a small change in the intermolecular vibrations.
CONCLUSION
The new method for the efficient generation of jet-cooled molecular complex ions is quite useful for the vibrational analysis of their electronic spectra. The phenoxy radical chromophore of the hydrogen-bonded complex cations of phenol-N(CH,), and -(NH,), has been confirmed and the phenol ion chromophore has also been shown in the phenol-H,0 complex ion. The difference in chromophore of the complexes has been discussed on the basis of the intermolecular stretching vibrations observed in the present work.
REFERENCES [1] J. Steadman and J. A. Syage, /. Am. Chem. Sot. 113,6786 (1991); J. Phys. C/rem. %, 9606 (1992). [2] R. J. Lipert and S. D. Colson, J. Chem. Phys. 89,4579 (1988). [3] K. Fuke, H. Yoshiuchi, K. Kaya, Y. Achiba, K. Sato and K. Kimura, Chem. Phys. Left. 108,179 (1984). [4] N. Mikami, T. Sasaki and S. Sato, Chem. Phys. Left. 180,431 (1991). [S] N. Mikami, S. Sato and T. Sasaki, Chem. Phys. Left. 202, 431 (1993). [6] N. Mikami, I. Suzuki and A. Okabe, J. Phys. Chem. 91, 5242 (1987). [7] N. Mikami, A. Okabe and I. Suzuki, J. Phys. Chem. 92, 1858 (1988). [8] G. N. R. Tripathi and R. H. Schuler, J. Chem. Phys. 81, 113 (1984). [9] H. Yu and J. D. Goddard, J. Molec. Strucf. (Theochem.) 233, 129 (1991). [lo] R. Liu and X. Zhou, J. Phys. Chem. 97, 9613 (1993); Chem. Phys. Lett. 207, 185 (1993). [11] H. D. Bist, J. C. D. Brand and D. R. Williams, J. Molec. Spectrosc. 21, 76 (1966). [12] X. Ripoche, I. Dimicoli, J. Le Calve, F. Piuzzi and R. Botter, Chem. Phys. 124, 305 (1988). [13] K. Walter, U. Boesl and E. Schlag, Chem. Phys. Left. 162, 261 (1989). (141 Y. Tsuchiya, M. Fujii and M. Ito, J. Chem. Phys. 90.6965 (1989). [15] R. J. Stanley and A. W. Castleman Jr., J. Chem. Phys. 94, 7744 (1991). [16] G. Reiser, 0. Dopfer, R. Linder, G. Henri, K. Mtiller-Dethlefs, E. W. Schlag and S. D. Colson, Chem. Phys. Left. 181, 1 (1991). [17] T. Ebata, M. Furukawa, T. Suzuki and M. Ito, J. Opt. Sot. Am. B 7, 1890 (1990). [18] D. H. Aue and M. T. Bowers, Gas Phase Ion Chemistry (Edited by M. T. Bowers), Vol. 2. Academic Press, New York (1979). [19] J. A. Syage and J. Steadman, J. Chem. Phys. 95, 2497 (1991). 120) M. Shiitz, T. Biirgi, S. Leutwyler and T. Fischer, J. Chem. Phys. 98, 3763 (1993).